1. Field of the Invention
This invention relates to lithography and, in particular, to lithography in device fabrication.
2. Art Background
Increased miniaturization has been a consistent objective in device manufacture. This objective is commonly expressed in terms of devices with stricter and stricter design rules. (The design rule defines the smallest dimension of a critical structure utilized in the device.) In the production of such devices such as semiconductor devices, selective patterning on the device substrate of structures with design rule dimensions is required. Such selective patterning is accomplished by forming a radiation-sensitive region on the substrate, exposing this region through its thickness to patterned radiation, and then selectively removing either the exposed or unexposed regions (typically through differential solubility) to produce the desired pattern.
As design rules become stricter, exposing radiation of shorter wavelength is required. For example, it is generally believed that deep UV exposing radiation (200 to 250 nm) will be required for design rules stricter than 0.5 .mu.m while vacuum ultraviolet (100 to 200 nm) or X-ray (0.5 to 100 nm) exposing radiation will be necessary for 0.3 .mu.m or finer design rules. However, with conventional approaches to lithography the use of shorter wavelength radiation presents additional difficulties. For example, typically a radiation sensitive material is exposed through its thickness. The image incident on the radiation sensitive material is in focus to a depth that strongly depends on the wavelength of the exposing radiation. For device structures involving 0.3 .mu.m lines and spaces, and radiation of wavelength 248 nm or shorter, the depth-of-focus is no greater than 1 .mu.m, but a radiation sensitive material generally of at least 1.2 .mu.m thickness is required to prevent total removal during pattern transfer. Therefore, at shorter wavelengths, where exposure through the thickness of the radiation sensitive material followed by differential dissolution is employed, some loss of resolution occurs.
A surface reaction approach to lithography significantly reduces resolution loss due to depth-of-focus. As described in an article by G. N. Taylor, et al. Journal of the Electrochemical Society, 131, 1659 (1984), a substrate such as silicon coated with a radiation sensitive organic film is selectively exposed to produce a pattern due to chemical change. The surface is then subjected to reagents that through chemical surface interaction produces formation of a mask material on the substrate in a pattern consistent with the initial exposure. (The substrate includes the device layers being processed and, if present, the radiation sensitive material.) The mask material is then utilized for selectively treating the underlying substrate in processes such as etching, lift-off or electroplating. Since only the change in surface chemistry is employed to produce the mask, depth-of-focus problems are significantly reduced.
Although these surface treatment procedures hold significant promise, they have not been extensively investigated. Chemistries suitable for exposures that must be done in a vacuum environment such as exposure to vacuum ultraviolet and x-rays have not been disclosed. Thus, a further elucidation of suitable chemistries for surface treatment lithography is desirable.